hello, ive got a pretty peppy setup and cant quite seem to get the studder out of it.... i have a older 350 .40 over with 65cc chambered afr heads (around 9.5:1) a t5, 4.10's and smaller than stock tires. i have a performer rpm gap intake with a 1" spacer and a demon mech 650, no choke with 70/74 jets in it. i run a 180 stat and i dont turn off the fans hardly. the cam was thumper 106 lobe sep. i had the fuel mapped the best ive ever had it last summer plugs looked good, i run premium and i run it at 14 degree's advance. i have a stock hei mech/vac distributor. with the medium mech advance springs installed. honestly it runs very strong just a real slight sputter if you stab it. could this be taken care of with simply a new ignition? or do i need to run the stiff/low advance springs in the distributor? or is there more to the carb tuning?

 

That's a lot of cam for low compression. I would first try getting the initial mechanical advance up to around 20*. Try on light spring and one medium spring in the HEI. Make sure the pump arm has no slack in it. you want fuel coming out of the squirters as soon as you move the throttle even a little.

Is the 1" spacer an open or 4-hole? My Air Gap/Demon combo hated the open spacers I tried. Big stumble off the line.

 

I had a hesitation issue when I had first setup my 350. I was running near stock cam with a little more aggressive profile/grind, victor jr. intake manifold, holley 650dp, and a stock distributor with a accel hei. I had extreme hesitation upon hard acceleration. The first thing I ruled out was the distrib it just was not the right setup the timing advance was all wrong. I switched over to msd hei mech/vac and a lot of the problem went away.

I still had some stumbling so I changed adjusted the float levels in the carb as well as the jets and changed over to a performer intake manni with no spacer, the vic jr. was just too much for the setup. After converting my problem was gone almost completely now.

The remaining problem was solved by upgrading to a fuel rail and a new 130?gph pump because the old pump was just not working as well as it once did.


The thumper cam is not to aggressive as long as the grind is right for the setup I had a thumper cam on a similar setup and it was find it gave a real aggressive sound an they are extremely street-able. They aren't made for low compression setups but 9.5:1 is hardly low compression my motor was 9.1:1 and it ran great.

Get a timing light. Bring the timing up 2 deg. at a time take a test drive see if it gets better. Also check the advance make sure its working properly usually that is the issue with hesitation. And as panch said check your jets action they are important.

 

Good advice so far. Do the timing curve first then head back to the carb.

Playing with squirter size and pump cams helps a lot sometimes you just need a touch more shot when you initially hit it. Most "carb" problems are usually ignition related if you catch my drift.

 

the spacer is an open style. i dont have a "big stumble"... it used to stumble before i dropped the mech advance down to the med. springs. before i had it at 16 degree's advanced with the light springs (high advance) and i got the marks on the spark plugs from having too much timing(forget what its called). so i know the advances are working. it runs best WOT honestly, just slight sputter when u stab it. i ran 68/76 in the carb and it didnt cut it, put the 70/74 and WOT at 6k is right at stoich/rich mark, runs a touch rich idlin at 800 rpm. i have 3.1l v6 fuel pump with it regulated at 6.5lb-7lb, never had a problem with fuel delivery

the pump arm i will check, and can c what the stiff advance springs will do. id b a blast to have a dino handy and c what everything i mess with does for the power

how do i experiment with the pump cam? ive messed with the adjustment on that and crankin that thing up for quicker fuel jet made things worse.

messing with the squirter size, would goin down slightly b best?

 

Use a four-hole spacer for better low RPM response.
Also, get the Demon carburetor book. It will explain all about adjusting your carb.

 

My experience has been that a "stumble" is almost always a lean condition or not enough ignition advance. Being too rich or having too much advance can cause problems too, but you just don't see it too often. For the pump cams, I would stick with a stock "pink" in the #2 hole. Make sure there is no slack in the arm at idle. Use a #32 or #36 squirter to start.

Lastly, get the primary and secondary throttle blades set up right (take the carb off). You don't want one open a bunch and the other closed all the way. This can cause that slight stumble at tip in. Once this is done, set your mixture screws for the highest vacuum reading at idle.

If none of this fixes your issues, you will need to look at the IAB's to help bring the fuel in sooner. As a test, you can raise the floats real high. That will bring the fuel in sooner. I spent almost a whole summer dialing in my Mighty Demon. I learned a lot and it was well worth it. I went from running 12.20's to 11.89 (3000' DA) and my idle is pretty clean. One last thing...you mentioned "marks" on the plugs. Was this just a heat band or actual deposits? A heat band is a good thing...deposits of molten aluminum are not!

The stock HEI's have around 22-24* mechanical advance built into them. That is too much. You need to limit that to about 18* and set your initial at 18* (for 36* total). You want it all in by about 3000RPM. I use one light and one medium spring from the MSD kit. That, combined with the stock weights gives me the right rate of advance. Also, the stock vacuum can needs a ton of vacuum to even start moving. It wont work for a performance engine. You need a can that starts advancing at about 10" and is all done by around 16". The can that comes with the GM performance HEI is a good match.

 

The poster above is right. That is exactly what you need to do. Try 1 light/ 1 med and set the timing at 36* total, with the advance disconnected and the carb port plugged at around 3000rpm. Let the initial timing lay where ever it ends up and then plug your vac advance back in and set the idle.

That should get rid of your issue, if not- get back to us.